Windows, U-Values and SHGC
U-Values
While we use R-values for measuring the thermal properties of insulation and the resistance to the movement of heat, we use U-values when talking about windows. A U-value is the inverse of an R-value. While a bigger R-value is better, the opposite is true of U-values.
For example, a common brick veneer wall with timber stud, plasterboard, and R2 insulation batts has a total system R-value of 2.24. The equivalent U-value is 0.45
The following list provides both the R and U values for different common window systems. Note that the system as a whole is rated, not just the glass. This is because frame material plays a big part in the thermal properties of a window due to the conductive properties of the material.
Single Glazing, Aluminium Frame: R0.15, U6.7
Single Glazing, Timber Frame: R0.19, U5.4
Double Glazing, Aluminium Frame: R0.21, U4.8
Double Glazing, Thermally Broken or Composite Frame: R0.28, U3.6
Double Glazing, Timber or PVC Frame: R0.33, U3.3
It’s worth noting that even thermally broken aluminium frame double-glazed windows will lose heat eight times faster than a typical brick veneer wall
Solar Heat Gain Coefficient (SHGC)
While R and U values measure resistance to conductive heat transfer from adjacent materials, usually air, SHGC measures the ability to block the direct radiant heat from sunlight.
“The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.” (Australian Glass & Window Association, 2018)
Why is it important? Because in cooler climates we want the window to be effective at keeping warmth inside the house, but also effective at allowing radiant heat from sunlight to enter the room. This means that windows with a low U-value and high SHGC are ideal in these climates.
An energy rating consultant can advise on achieving the best balance for the location in which you’re working. We work with an energy-assessor on every project as a non-negotiable.
Window dressings and shading
External window shading devices are excellent for reducing sunstrike to the window and therefore reducing unwanted heating inside. Optimised fixed shading, horizontal louvres, vertical louvres, and operable blinds all have useful applications depending on the orientation and location of the window.
Internal window dressings are useful during winter and act as insulation against the window. Good window dressings that reduce convective airflow past the glass can be as effective as an additional layer of glazing (ie making double glazed windows as effective as triple-glazed windows when the blinds or curtains are closed overnight). However, note that they are not very effective at keeping the sun (and heat gain) out in summer because the sun has already passed through the glass into your home by the time it hits the curtain. ‘Good’ window dressings include curtains with a pelmet over the top, blinds that sit flat against the architraves (like Roman blinds) or thick blinds that fit tight within the wall opening (cellular blinds).
Low-e coatings
In cooler climates where a home is designed with solar passive principles in mind, high solar transmission low-e coatings can be highly beneficial. These coatings still allow the sun’s energy to enter your home (and warm up surfaces that it hits), but it will insulate against the resulting heat escaping back out through the window. In other words, it makes a negligible difference to the warming power of the winter sun but improves the insulation capability of the window. For a more thorough and scientific explanation, see www.vitrowindowglass.com
The bottom line is to remember that there are many complex options when it comes to windows and they may be appropriate or financially sensible in different climates, designs, and orientations. This is exactly what a skilled energy assessor can advise on, and while we have extensive experience and confidence in the application of these window options, we still insist on having an energy assessor confirm the application for each project because when you’re investing in such a large amount in the complex machine that is a home, why wouldn’t you want to make sure you’re doing it right?
Case study:
On our Tumbarumba house, by requesting alternate window quotes and working with an energy assessor (Light House Architecture and Science) we assessed that adding low-e to the uPVC windows would cost an additional $930 upfront while resulting in a 14% reduction in heating and cooling load, with a projected saving of $3640 in heating and cooling bills over 20 years (assuming 3% electricity cost increase each year).
On this project, we tested pricing on a range of thermally broken aluminium windows, and uPVC windows, which all sat within a price range of $27,800-$30,500 supply, at the time.
Double glazed, but non-thermally broken aluminium would have been much cheaper, but thermal modelling indicated that compared to the basic uPVC windows (even without low-e) the heating and cooling requirements of the building jumped up by more than a third. This was estimated at almost $10,000 additional heating and cooling energy costs over 20 years. Worth considering is the likely additional condensation challenges and general reduced ‘comfort’ levels that would have been experienced in this project, which is why as a rule we generally never consider basic double glazed aluminium windows in our high performing designs.
Now, of course, these numbers can vary hugely depending on your project intricacies, and this information is really just to demonstrate what’s possible when weighing up the options for your own project. Working with an architect who can connect you with and coordinate an appropriate team for your project can deliver similar informed options and results for your own build.
Where to find more
The information above is extracted from my book, 101 Things I Didn't Learn in Architecture School; And wish I'd known before my first job.
While the book is written for students and graduates of architecture, it is also an excellent handbook for anyone embarking on their own project. We provide a free copy with every on-site briefing and feasibility meeting, or you can purchase your own copy here.
This article from Renew is an excellent summary and provides further information on tints and other options not covered above
My knowledge of this subject comes from years of working under Jenny Edwards, building scientist and Director at Light House Architecture and Science. Her website includes further excellent resources and case studies on this topic. See below for two of Edwards’ articles, referenced above.
Another great resource on basic principles in Australia is the YourHome.gov website which is free to access.
References
Australian Glass & Window Association. 2018. “Frequently Asked Questions.” www.wers.net/werscontent/faqs#Q3
Edwards, Jenny. 2018. “Windows 101 – R U Confused.” Light House Architecture & Science. www.lighthouseteam.com.au/journal/windows-101-r-u-confused.
Edwards, Jenny. 2018. “Windows 102 – Ratios And Coefficients.” Light House Architecture & Science. www.lighthouseteam.com.au/journal/windows-102-ratios-and-coefficients.